1. Field of the Invention
The present invention relates to a glass plate for e.g. a cover glass or a glass substrate of a display device, typically a mobile device such as a cell phone or a personal digital assistance (PDA) or a small display device such as a touch panel, a plate glass used for production of such a glass plate, a process for producing such a plate glass, a cover glass and a display device.
2. Discussion of Background
In recent years, for mobile devices such as cell phones and PDA, use of a cover glass (protective glass) for protecting a display and improving appearance, is increasing.
Weight reduction and thickness reduction are required for such portable digital devices. Therefore, a cover glass used for protecting a display is also required to be thin. However, if the thickness of the cover glass is made to be thin, the strength is lowered, and if a display device is dropped at a time of using or carrying it, the cover glass itself may sometimes be broken. Therefore, there is a problem that the cover glass cannot accomplish the original object to protect display devices.
In order to solve the above problem, it is conceivable to improve the strength of the cover glass, and as such a method, a method to form a compressive stress layer on a glass surface is commonly known.
As the method to form a compressive stress layer on a glass surface, typical are an air quenching tempering method (physical tempering method) wherein a surface of a glass plate heated to near the softening point is quenched by air cooling or the like and a chemical tempering method wherein alkali metal ions having a small ion radius (typically Li ions or Na ions) on a glass plate surface are exchanged with alkali ions having a larger ion radius (typically K ions) by ion exchange at a temperature lower than the glass transition point.
As mentioned above, the thickness of the cover glass is required to be thin. If the air quenching tempering method is applied to a thin glass plate, the temperature difference between the surface and the inside tends not to arise, and it is thereby difficult to form a compressive stress layer, and the desired property of high strength cannot be obtained. Therefore, a cover glass tempered by the latter chemical tempering method is usually used.
As such a cover glass, glass having soda lime glass chemically tempered is widely used (e.g. Patent Document 1).
SiO2—Al2O3—Na2O glass other than the soda lime glass has also been proposed as a cover glass, and as a process of such glass, a downdraw process or an overflow downdraw process has been assumed (e.g. Patent Document 2).
Patent Document 1: JP-A-2007-11210
Patent Document 2: JP-A-2009-57271
A cover glass or the like for a mobile device may have a hole having a function as a speaker or the like, or it is preferred to have a complicated shape in view of the design. Accordingly, when a plate glass for a cover glass is processed to form a glass plate for a cover glass, it is subjected to a complicated process such as drilling or scribing a curve to be formed into a glass plate having a final shape in many cases.
If chipping occurs when such a plate glass (so-called raw plate glass) is processed, latent flaws are also formed at the same time. Such chipping formed at the time of processing before chemical tempering treatment has not conventionally been problematic. However, in a case where a glass plate which has remaining latent flaws which have been formed at the same time as chipping by processing is chemically tempered, if the latent flaws are so deep that they are beyond the compressive stress layer, the improvement in the strength of the glass plate may be insufficient. Further, if the latent flaws are present in an interior tensile stress layer of glass, cracks may expand by themselves and the glass may voluntarily break.
Further, in a case where chemical tempering treatment is carried out while glass dust formed by chipping is attached to the surface of the glass plate, only that portion is not tempered and may cause a drawback such as a dent or warpage. That is, the chipping may cause a product failure other than the latent flaws, which may lead to a decrease in the strength.
Further, substrates before processing are getting larger so as to improve the productivity, and may have a G4 (680×880) size or a G5 (1100×1300) size.
In a case where such a large substrate is processed, as the one side tends to be long, formation of deep latent flaws is highly possible. Further, as the substrate gets larger, it is heavier and is likely to bend, and the probability of breakage from the latent flaws tends to be high.
Accordingly, the probability of chipping is increased by the increase in size of the substrate, and the breakage failure of glass may be increased in the substrate processing step.
Such problems due to the chipping are more serious due to an increase in the demand for a reduction in the thickness of a cover glass accompanying need for weight saving of e.g. mobile devices. That is, if the thickness of a cover glass is reduced form 2 mm to 1 mm, the strength is reduced to one quarter, and the above-described problems are more serious.
Further, such a cover glass is suitably formed by a downdraw process such as a downdraw process or an overflow downdraw method in many cases, but such a method is not necessarily applicable to mass production.